Anti tie down palm buttons

[Tim]

PAUL,
The purpose is to make sure that one of the buttons are not "TIED" down. This would allow the machine to operate with just one button or one hand which is not good. Yes, you can just use one timer.

      pb1

------| |-------|-------------------------(T1)-----

                |

                |

      pb2       |

------| |-------|



      pb1       pb2      T1

------| |-------| |------|/|---------------(RUN)-----

 

[elevmike]

Tim,

My previous post here uses just one timer. I Think it works as required.

Mike.

 

[Rick Densing]

It has more than required. The other question is do you want a pulse or a continous output from the buttons? Also the accepted time is usually .5 sec.

Follow the links, it is all covered there.

 

[Tim]

Tim,

My previous post here uses just one timer. I Think it works as required.

Mike.

This topic has been discussed so...so...much.
You got some things in your code that are not needed. You could easily clean this up to only a few steps. The whole idea is the buttons need to be depressed within a certain amount of time. I believe the time is usually a 1/2 second. I'm not saying your code doesn't work, but just needs cleaned up. I also don't like the fact of what if both of your PB's stuck down, or the operator doesn't release both of his hands off of them. Without knowing the details of the machine its hard to say what would be acceptable.

I know that there are several ways around this, but I think the simplist way is the best and as Rick stated it all might depend on the start sequence. Paul didn't say Anti-Repeat, but if you allow any one of the PB's to use the timer at anytime, then this can also give you the ANTI-REPEAT as well.
Tim

 

[elevmike]

Tim,

My code will disable the RUN if both buttons are not pressed within the timer preset range. However it will hold the RUN if the buttons are pressed within the timer preset.

Your code will disable the RUN if both or one or the other button after the timer times out.

Based on my understanding of how ATD is supposed to work, I dont understand how your code would work properly.

Additionally your correct that I dont address the anti repeate issue, as I dont know how his machine will tell (feedback) the PLC when the cycle is compleated.

Mike.

 

[ndzied1]

the following is posted purely for educational purposes and is not intended to be used in any real application


elevmike wrote:

Just out of courisoty, would the following hard wire be considered legal? Using my code above will monator the buttons and provide the output to run the press. However if the PLC output locks on the press still will not run unless both the buttons are pressed.

I understand the timer function would be lost in event of a PLC output lock-on, but still the press wont start without the buttons on.

                

                                  PLC

                                 OUTPUT

                                 COMMON 

                                   *     PLC

    LH BUTTON        RH BUTTON     |    OUTPUT        

-----*^*-------*-------*^*---------*-----||-----*-------(press)

               |                   |

               *                   *         

            PLC INPUT 1        PLC INPUT 2                      

Just to be clear, this is a question, not a suggestion.

Thanks,

I have just had to read NFPA 86 which is concerned with Furnace and Oven control and might shed, at least their view, on this situation.

It states, and I'm paraphrasing here, that you can put contacts from a non-safety rated PLC into the safety string so long as they are not connected to the control power source. My reading on that was isolated realays would be what you could use there. Note, that these contacts are in addition to all the hard wired safety contacts which need to be in the string and not in place of any of them. This allows for the plc to enable and stop the burner but the PLC is in no way used to shut down the burner for a field device which has gone out of its safe operating parameters.

In that light, (and now this is a hypothetical of a hypothetical) your connection to the PLC output common could be problematic. Also, as I'm looking at your connection to PLC inputs, it allows for interposing relays on the safety field devices which seems to be how the signals are picked up by the PLC for alarming and display to the operator. A big point is that only one device can be connected to each relay. Like most specs, it does contradict itself on the rating of these relays and in the same sub-section says they have to be rated for combustion duty or that they can be general purpose... go figure.

Other codes can easily contradict this one so this is only one take on how to marry safety items with non-safety PLC's.

This is a 2003 code and it does allow for safety rated PLC's. To meet the code, the PLC must be rated for combustion management so just any old safety PLC will not do. It also stated that all safety related code must be kept separate from other code. Here my interpretation is that this prevents you from, for instance, de-bouncing a signal that should cause a safety shutdown immediately.

When at the Design Show in Chicago last month I investigated PILZ which does have a safety PLC. They have different application packages (think function blocks) which are rated for different things and they do have a burner management package. You actually pay for these application packages on a per/PLC basis. I even got to meet Thomas Pilz but that paled in comparison to getting to meet Peter (Dr. Loop) Nachtwey at his Auto Tuning presentatoin....

 

[elevmike]

Thanks Norman,

I thought it would never come.

Thnaks very much.

Mike

 

[JesperMP]

"elevator mike"

"...would the following hard wire be considered legal ?"

I would have to say no, under the conditions that your proposed circuit is supposed to ensure safety from a machine that could maim a person if the circuit fails.

You are using the PLC to programmatically block the case that one or both buttons are tied down. You just CANNOT use a standard PLC for safety functions.

Just to give you an idea what the safety relay (that PILZ and others make) really is:
It consists of two separate channels. Each channel is monitored by a CPU. The two CPUs also monitor each other. The two CPUs are not the same type, this to ensure that a manufacturing error or programming error doesnt defeat the safety on both channels. The device is then testet and certified by an approved testing agency.
You then have to use the relay exactly as it is supposed to according to the documentation.
You also cannot use just any combination of pushbuttons for the two-hand control. It must be an approved type etc. etc.

Your diagram and code would probably work fine, but the law doesnt allow it.

It is a completely different story that the correct procedures arent followed everywhere.

And two-hand control is from a time where you didnt have light curtains. The latter is being used more and more these days, it can save cycle time, is safer etc. etc. But it is not everywhere that you can fit a light curtain, two-hand control still has its use for some.

 

[Tim]

If you need to hold your hands on the buttons use this code. If you need to just hit the buttons once use the first code I posted. Do a search on this topic as it has been covered.

 

[bob1371]

Prior to joining my present company, someone convinced them to install Banner safety relays and opti touch start buttons. Last month I recieved a letter from Banner saying they have a recall on the model I use. Well we just finished replacing 85 of those damn things. Whoever convinced them they were needed was a hell of a salesman. All these machines are also equipped with light curtains. They are small assy machines that will in no way cause serious injury.
Although adding these didnt hurt anything (maybe the budget) I stongly suggest evaluating needs and requirements first. Personally I think with the light curtains in place and anti tiedown logic in the plc is plenty.

Bob

 

[elevmike]

Tim,

You second drawing now makes sense to me. You missed the RUN contact that jumped out the timer contact the 1st code you posted. However your correct, as I see it now my code was too elaborate.

Thanks.

Mike

 

[Kroz]

Why would you want to use one timer? When it comes to safety, wouldn't you want to use two for double redundancy?

 

[elevmike]

Safety?, Who's talking about Safety?

I was just being hypothetical. Hopefully the thread starter was too!

Mike

 

[Kroz]

wasn't knocking you elevmike, just wanted to put the idea into the thread starters head

 

[elevmike]

Kroz,

No offence taken, Just kidding..

Your absolutely right. You cant say too much about safety.

Mike